The process of drilling an oil/gas well liberates gases from the formation, which are carried to the surface by the drilling fluid. The measurement of this liberated gas at the surface is a well-established method of qualitatively indicating the gas/oil content of the formation being drilled. However, because of the limitations of methods of gas-sampling from the fluid, the current and prior methods for measurement preclude the direct quantitative measurement of these gases as a percentage by volume of the drilling fluid.
One current sampling device is known as a “gas trap” or “mechanical agitator”. Gas traps are unable to measure quantitatively gases in drilling mud. A gas trap is, in essence, a mechanical converter that converts a percentage of gas in the mud to a percentage of gas in air, which can be directly measured. Dissolved gases are broken out by mechanical agitation, and removed by gas sampling equipment through a sample line to an analysis device. As long as the total gas percentage in the drilling fluid remains within the soluble range, air will be re-introduced into the trap at approximately the same rate as the sample is drawn out. In this case, the percentage of gas measured will be proportional to the actual percentage in solution, depending on the efficiency factor of the trap. As the gas percentage in the mud increases, however, it will reach a point where the mud is saturated with all the gas that can be held in solution, and the remaining gas will exist as free bubbles forming froth in the drilling fluid. These bubbles will displace air in the trap canister. This falsely increases the apparent concentration at the sampling point. Current gas-trap based measuring systems exhibit large non-linearity in response due mainly to this required conversion between gases in solution and gases in the form of froth in the liquid.
The gas trap is not the only problem with the current system of gas measurement. Since the gas analyzer is remote from the trap, a sampling system is required to draw the gas sample from the trap to the analyzer through small diameter tubing. This introduces other variables into the analysis, such as the sample draw rate, temperature effects on the sample as it passes through the tubing, integration of sample peaks by long lengths of tubing, delays caused by the sampling tubing length, etc. These, along with a myriad of other variables in the analysis process, have substantially precluded accurate quantitative analysis of gas in drilling fluids. This has made it difficult or impossible to form quantitative comparisons of data between similar oil/gas wells in the same field, or from the same formation in different fields. The development of a method for accurately determining well bore gas concentration data without the necessity of expensive intrusive measurements such as Drill Stem Testing has been a goal of the industry for many years. Such a method would permit the comparison of surface-gathered data from different wells to determine the relative oil/gas content of a given formation.
Gas permeable membrane probes, such as the GasWizard™ instrument (Datalog Technology Inc.: Calgary, Alberta, Canada) are now available that are capable of determining the concentration of a gas directly in a liquid. Thus, the gas permeable membrane probes address the problems of gas detection using a gas trap and remote analyzer by eliminating the conversion step from gas in mud to gas in air. With the elimination of this conversion, it is possible to directly read the gas concentration in the mud system. It also eliminates many of the problems incurred by remote sampling of the gas stream.
A gas permeable membrane probe consists of a flexible probe with a gas permeable membrane at its end. The gas permeable membrane is immersed in the drilling fluid at or near the point where the fluid exits the well bore. This probe is attached to a measuring instrument in a watertight, explosion proof enclosure, which contains the necessary microprocessor based electronic and pneumatic controls for operating the probe, along with a compact gas analyzer for the measurement of gases absorbed at the probe. The device outputs collected data via a serial communications link which may be attached to a remote computer for data logging and additional analysis.
Gas permeable membrane probes are based on gas permeable membrane technology described in U.S. Pat. No. 5,317,932 by Westlake and Wolcott. The use of the probe to sample oil-drilling muds is described in U.S. Pat. No. 5,469,917 of Wolcott.
While the use of gas permeable membrane probes has improved the measurement of gas concentrations in drilling mud, it is still difficult to measure quantitatively the concentration of gas in drilling mud. In particular, since the probe is calibrated in either air or liquid, the probe cannot correctly measure gas concentration where the drilling mud is saturated with gas and some gas is present both in solution and as froth in the drilling mud.
A method is needed for measuring accurately and quantitatively the concentration of a gas in a liquid, such as drilling mud.